The multi-energy X-ray security inspection system is a new security inspection system developed on the basis of the single-energy X-ray security inspection system. In addition to providing shape and content of inspected objects, the system can provide information about effective atomic number of the objects to distinguish organic and inorganic objects, and display them in different colors on a color monitor to help an operator identify them. During inspecting and scanning a package, the operator wants to rapidly mark one target, for example, each 3 to 5 seconds. This requires automatic removal of any blank region in data preprocessing phase, so that a user can pick up a desired region including the target without dragging a mouse for a long distance to moving a marking arrow across the invalid data regions.
The Ray Casting method is one of the existing volume rendering techniques (Max N. Optical models for direct volume rendering. IEEE Transactions on Visualization and Computer Graphics, 1995, 1(2): 99-108) that are mostly used. In this method, a ray is emitted along a fixed direction from each image pixel, and the ray penetrates the entire sequence of images. During this process, the images of the sequence are sampled and classified to obtain color values, and the color values are accumulated according to a ray absorption module until the ray passes through the entire sequence of images. The finally obtained color values are colors for a rendered image.
In the Ray Casting method, an often-used accelerating technique is Empty Space Skipping (Wei Li, Klaus Mueller, and Arie Kaufman. Empty space skipping and occlusion clipping for texture-based volume rendering. In Proceedings of IEEE Visualization, pp. 317-324. Los Alamitos, Calif.: IEEE Press, 2003). The existing transparent region skipping methods are primarily used for acceleration in case of using one-dimensional transfer functions. Such methods have poor accuracy and low speed. Further, these conventional methods are used in case whether the transfer function can be adjusted in real time.